1. Technical Field
This invention relates generally to piston assemblies, and more particularly to wrist pins, pistons, connecting rods and piston assemblies therewith and to their respective methods of construction and assembly.
2. Related Art
Internal combustion engines are known to have piston assemblies including a piston, a connecting rod and a wrist pin. The wrist pin attaches the piston to the connecting rod for translation of the piston within a cylinder bore in response to rotation of a crankshaft and combustion within the cylinder bore. Typically, the piston has a pair of pin bosses with axially aligned cylindrical pin bores configured for receipt of cylindrical ends of the wrist pin. The connecting rod has one end configured for attachment to a crankshaft and a small end with a cylindrical small end bore configured for receipt of the cylindrical wrist pin therethrough. Accordingly, the wrist pin couples the connecting rod to the piston.
In an effort to reduce friction between the wrist pin and the pin bores and the wrist pin and the small end bore of the connecting rod, typically a suitable bearing material is incorporated in the pin bores and the small end bore. Although the bearing material can assist in reducing friction, inevitably wear results to the bearing surfaces of the wrist pin and the respective bores. The wear generally results due to a lubrication film breakdown between the bearing surfaces. The lubrication film breakdown results generally from the excessive pressures generated between the bearing surfaces, the relative rotating and/or oscillating movement between the bearing surfaces, an insufficient supply of lubrication reaching the bearing surfaces and from exposure of the bearing surfaces and the lubrication to high heat.
In addition to ultimate failure of the bearing surfaces, another undesirable phenomenon typically results from the breakdown of lubrication. Noise typically is generated between the bearing surfaces of the piston pin bores, the wrist pin and the connecting rod when the lubrication film is diminished. The lubrication film breakdown results in excessive clearance between the components, and thus, insufficient damping occurs between mating bearing surfaces.
In an effort to minimize the negative consequences discussed above, considerable care is generally taken during manufacture of the bearing surfaces of the piston pin bores, the connecting rod small end bore and the wrist pin. In order to minimize breakdown of the lubrication film, more clearance needs to be incorporated between the respective bearing surfaces. However, in order to reduce the onset of noise, the clearance between the respective bearing surfaces needs to be minimized. As such, in order to attain the enhanced lubrication and reduced noise generation desired, precise diameters and surfaces finishes of the bearing surfaces over a relatively narrow tolerance range need to be attained, such as in tightly controlled machining operations, e.g., grinding operations. As such, the costs associated with the manufacture of these parts is relatively high in comparison to parts requiring less precision. Although the manufacturing processes have improved, thereby allowing for more precise fabrication over narrow tolerance ranges, the ever increasing demands of modem engines, including higher cylinder pressures and engine speeds, and the ever increasing environmental regulations for reduced engine emissions and increased fuel economy, make it difficult or impossible to attain clearances and sufficient surface finishes between the respective bearing surfaces to combat the operating environment of the modem engine while in use, such as extreme pressures and temperatures, for example.